Projection films are used with rear projection screens and monitors for transmitting an image generated by a projector or the like located at the back of the screen to the viewer on the opposite side of the screen. The viewable image is generally affected by the amount of light transmitted through the screen. Generally, the construction of rear projection screens and monitors limit the amount of light transmitted through the screen or monitor to the viewer. It is thus desirable to have constructions that provide increased light throughput.
Generally, rear projection screens and monitors suffer from poor angularity resulting in difficulties in viewing a projected image unless the viewer is facing the viewing area of such a screen or monitor head-on. Angularity is the term used to describe the ability of a viewer to see a good image from the screen at angles other than those which are normal to the surface of the viewable screen. For example, as the viewer moves to the side of the viewing screen, the image quality is generally decreased. It is desirable to have projection screens and monitors which have improved angularity.
Projection films can be generally characterized by their performance parameters: resolution, gain, transmission, ambient light rejection, half gain angle, and contrast. All of these parameters are generally determined by the structure and materials used in construction of the projection film. The resolution of the projection film is determined, at least in part, by the size and spacing of minute transparent particles, commonly referred to as microspheres, (e.g., the more microspheres contained on a substrate generally means increased resolution of the projection film). The peak gain is a measure of the intensity of the light transmitted from the rear of the film to the viewer side of the film as a function of angle measured from normal incidence and is normalized with respect to a lambertian diffuser. It is determined at least in part, by the refractive index of the microspheres and the surrounding material. Ambient light rejection and contrast are affected by absorption of an opaque layer, which typically is used to embed the microspheres. The viewing angle of a particular film is defined as the angle at which the peak gain is reduced by 50%. Such angle is commonly referred to as the “half-gain angle”. Contrast is a measurement of the difference in light intensity between the brightest white and the darkest black reproducible on the viewing side of the projection film.
An exemplary prior art projection film 10 is illustrated in FIG. 1. Typically, the prior art projection film 10 includes an optically clear support layer 12, a light absorbing layer 14 deposited over the optically clear support layer 12, a plurality of microspheres 16A-16F embedded in the light absorbing layer 14, and may optionally include a conformable coating layer (or gain modification coating layer) 18 deposited over the microspheres 16A-16F and/or the light absorbing layer 14. In operation, light is projected from a light source (not shown) and transmitted through the conformable coating layer 18, the microspheres 16A-16F, and the optically clear support layer 12 to the viewer. The light absorbing layer 14 absorbs the light not transmitted through the microspheres 16A-16F. In addition, the light absorbing layer 14 also absorbs ambient light incident on the optically clear support layer 12 from the viewer's side in an effort to minimize reflections to the viewer.
The conformable coating layer 18 provides a predictable light gain profile based on the refractive index of the microspheres and the refractive index and thickness of the conformable coating layer 18. As shown in FIG. 1, the conformable coating layer 18 is generally of substantially uniform thickness across the exposed surface of the microspheres 16A-16F. This uniform thickness modifies gain as well as improving the focus of the light entering the film in an effort to maximize the total light transmission through the pinhole at the light exit surface of the microsphere 16A-16F.
As shown in FIG. 1, when the microspheres 16A-16F are relatively uniform in diameter, the microspheres 16A-16F generally penetrate through the light absorbing layer 14 and into the optically clear support layer 12 at a uniform depth. When the microspheres 16A-16F have varying diameters, however, as shown in FIG. 2, the conventional method of embedding microspheres 16A-16H causes deeper penetration of the larger microspheres (e.g., 16B, 16D, 16F) through the light absorbing layer 14 and into the optically clear support layer 12. Smaller beads (e.g., 16A, 16C, 16E, 16G, and 16H) can show minimal or no penetration through the light absorbing layer 14, which essentially minimizes or eliminates light transmission through the smaller microspheres. Another drawback associated with conventional processing of microspheres having varying diameters is that the light absorbing layer 14, which is generally made up of a black thermoadhesive coating, may wick up the sides of the smaller microspheres (e.g., 16A, 16C, 16E, 16G, and 16H) during conventional processing and cause thin spots between the microspheres. These thin spots allow light to pass through the thinned interstitial coating instead of being focused through the microspheres. This leads to the appearance of bright spots between the microspheres. This defect is known as “punchthrough”. Punchthrough also has undesirable effects on the optical properties of gain and half-gain angle. Another drawback is that the thermoadhesive black coating of the light absorbing layer 14 can be transferred to the rolls of the embedding apparatus and re-transfer portions of the thermoadhesive black coating to the light entrance surface of the microspheres. Thus, there is a need to overcome the drawbacks set forth above and provide projection films having improved microsphere alignment on the light exit surface and projection films with variable gain within a single projection film.